1. Field of The Invention
The present invention relates to a process for fabricating a non-foamed surface layer on foamed polyvinyl chloride products by a simple extrusion of polyvinyl chloride resins using a foam-compounding method to which an anti-plasticization phenomenon applies without the modification of conventional single-screw extruders. Foam-structured materials of polyvinyl chloride resins have commercially become the subject of interest ascribable to the superior physical properties thereof.
2. Description of The Prior Art
Various techniques for rigid polyvinyl chloride foam products have been proposed in the prior art such as U.S. Pat. No. 3,706,679, and European Patent Nos. 0,037,308, 0,039,117 and 0,039,118. Nevertheless, most of those prior art techniques were concerned with the preparation of rigid polyvinyl chloride foams having a thickness of less than 0.2 mm. Physical properties of the commercially available rigid polyvinyl chloride foam products are suitable for use in various purposes, but the products should still be improved with respect to their surface characteristics such as brightness, scratch resistance, etc. These drawbacks are deemed to be ascribable to the thin thickness of the non-foamed layer.
The methods for the preparation of the rigid polyvinyl chloride foam products having a thick non-foamed layer are disclosed in U.S. Pat. Nos. 3,764,642 and 3,825,637. However, the former requires a modified extruder which must be equipped with a torpedo and the latter must employ an injection molding of foam cored sandwich structure method; both resulting in an increase of the production costs.